1. Field of the Invention
This invention relates to continuous phase modulation in general. More particularly, the invention relates to a binary modulation and differentially coherent detection transmission system.
The differentially coherent detection of a continuous phase signal is a particularly interesting solution in situations where carrier recovery is difficult, due to the simplicity of the receiver which is derived from it and to its performances. The performances obtained for this type of detection present a slight fall off by comparison with the performances obtained with a coherent detection requiring a receiver of greater complexity.
Furthermore, continuous phase modulation with a modulation index equal to 0.5, known to those skilled in the art as MSK (Minimum Shift Keying) modulation, has the advantage of spectral compactness of the modulated signal notably by comparison with four-state phase modulation (PSK).
These two reasons, inter alia, result in MSK modulation with differentially coherent detection being considered for use notably for digital data connections between mobile radio terminal equipment in the project for a telecommunications by satellite system (MSAT) for mobile terminal equipment.
In this context, it is desirable to supply transmitter/receiver equipment providing increased performances by comparison with the existing transmitter/receiver equipment.
2. Description of the Prior Art
M. K. SIMON and C. C. WANG describe in the article "Differential Detection of Gaussian MSK in a Mobile Radio Environment", IEEE Transactions on Vehicular Technology, Vol. VT-33, No. 4, Nov. 1987, a transmission system with GMSK (Gaussian MSK) modulation and differentially coherent detection using in a receiver a Gaussian type impulse response receiver filter, in which a message comprised of a string of symbols to be transmitted from the transmitter towards a receiver is differentially coded in the transmitter into a string of coded symbols in such a way that a coded symbol of rank k in the string depends on the corresponding symbol of same rank in the message and on the coded symbol of rank k-2. A delay of duration 2 T, where T is the rate period at which the symbols are issued, is provided in the receiver so as to delay a received coded signal carrying the string of coded symbols. The received signal and the delayed received signal are multiplied so as to deduct by low-pass filtering from the product obtained a signal of which samples at instants kT are compared to a non-zero amplitude threshold so as to decide the values to be attributed to each of the message symbols.
This GMSK modulation transmission system reduces the probability of decision error with regard to the received symbols in comparison with other known equipment. A better aperture of the eye chart is obtained subsequent to the differential coding carried out according to which a k.sup.th coded symbol depends on the (k-2).sup.th coded symbol and not on the (k-1).sup.th as is usually the case. However, in this system, the intersymbol interference that is inherent to MSK modulation is not eliminated from the received signal. A greater reduction of the probability of decision error can be obtained by a transmission system in which the noise intersymbol interference would be eliminated from the samples of the received signal.